1. Field of the Invention
The present invention relates to a lithium ion battery, and more particularly to a prismatic lithium ion battery having a structure able to suppress swelling.
2. Description of the Related Art
A lithium ion battery is used as a power source for portable electronic appliances, because the lithium ion battery can store more energy in a smaller volume and with less mass. The lithium ion battery is divided into a can-type battery having a case receiving an electrode assembly made of a constant shape metal can, and a pouch-type battery having a case receiving an electrode assembly made of a thin, flexible pouch.
The lithium ion battery employs one of cobalt acid lithium, nickel acid lithium, manganese acid lithium, and lithium salt of an oxide of a solid solution containing at least two of cobalt, nickel and manganese, which can occlude and discharge lithium-ion, as a positive active material. Various of carbon structures, which can occlude and discharge lithium-ion, are used as a negative active material. The positive active material and negative active material are formed into a binder, solvent, conductive agent and slurry are coated on the surface of a collector, and thus an electrode is formed.
A separator is installed between positive and negative electrode plates, and an electrolyte which makes the current flow through the medium of the lithium ion between the two electrodes is employed.
Because the lithium battery has a high charge/discharge operation voltage, the side reaction between the electrolyte and the electrode material may occur when a high voltage is applied, or the side reaction between the electrolyte and the electrode material may occur at high temperature due to heat generated during battery operation.
For a graphite negative electrode, the volume of battery increases when the battery is charged, and the volume increase results in expansions of the electrode assembly and the battery.
The gas generation and internal pressure increase are mainly due to anomalous reaction in the battery. If the anomalous reaction persistently happens or above a certain level, the safety of the battery is decreased.
Even the internal pressure increase and the expansion of the battery happens by slow degrees during a normal operation, the expansion is irrelevant to the safety. The structure of the battery is designed to operate the battery in case of such an expansion of the battery during normal operation.
Even during a normal operation, the expansion of the battery may cause operational problems of the battery. For example, when a rectangular battery is selected due to a thin installation space, the expansion of the prismatic lithium ion battery degrades the adaptability in the electronics, and it may even cause the breakdown of the electronics and the fracture of the battery. The prismatic lithium ion battery is lithium ion battery with a prismatic cell, prismatic cells have narrow and slimmer shapes and are most common in the lithium battery family.
Furthermore, even if the battery is expanded with ease, the internal pressure increase is delayed due to the battery expansion for a long time, and it is possible that a thermal runway or an ignition is occurred before the safety device is activated.
Therefore, in order to prevent the problems above, an improved case design of the battery is needed to prevent the case from swelling or increasing in thickness in case of a certain degree of the electrode expansion or gas generation.